In the area of filter media technology, needle-felted filter fabrics have long been used because of the numerous advantages which they offer. One of these advantages is that, since it is possible to have a random arrangement of fibers in a needle-felted structure, there is an extremely large number of pores available for filtration. Further, such a structure allows a large fiber surface area to occupy a relatively small area of fabric. In addition, it is relatively easy to produce needle-felted filter media which have excellent pore size uniformity as well as desirable dimensional stability.
Due to the aforementioned characteristics, presently there are available large numbers of needle-felted filter media which meet a large number of diverse applications. For example, when filtering temperatures do not exceed 215.degree. Fahrenheit, wool felts can be used to filter gases and the like. If filtration is to be done at moderate temperatures up to 270.degree. Fahrenheit, felted filter fabrics can be made from needling acrylic fibers. Filter fabrics having higher temperature resistance are known; such fabrics usually incorporate a mixture of various mineral and/or synthetic fibers. In this connection, reference is made to German Pat. No. 2,232,785 which relates to a needle-felted filter fabric utilizing an asbestos/glass fiber mixture in the batt layer and a mineral fiber base layer. However, such materials are believed to have poor reproducibility due to the difficulties involved in controlling the pore size uniformity of the glass/asbestos mixture during the needling and resin-treating processes involved in making such filter fabrics. Further, such an asbestos/glass mixture in a filter fabric is thought to present potential health hazards, as asbestos fibers may be released into the environment during the production of such filters.
Another material which has been found useful in high-temperature filtration is polytetrafluoroethylene; however, it is not suitable for all filtering applications due to its high cost and lack of dimensional stability at elevated temperatures.
Glass fibers are known to be resistant to most corrosive environments and to possess dimensional stability at elevated temperatures. These properties have permitted glass fibers to be incorporated into a wide variety of filtering media. In this connection, reference is made to Silverman, et al. U.S. Pat. No. 2,758,671; Dennis, U.S. Pat. No. 3,262,578; Kinsley, U.S. Pat. No. 3,920,428; and Taylor, U.S. Pat. No. 3,061,107. However, all of the aforementioned patents are nonneedle felted structures; and hence, up to the present it has not to our knowledge been possible to produce a needle-felted filter fabric comprised of an all glass fiber batt or surface layer and a mineral fiber base or underlayer. Such a structure has long been desired since it would offer the advantages of both needle-felted filters and glass fibers.
Glass fibers that are embodied in a structure as described herein possess a number of excellent properties which make them desirable for a filtration material. For example, it is known that glass fibers are relatively inert and, therefore, are resistant to alkali as well as most acid solutions. Also, glass fibers are stable over a wide range of temperature and humidity conditions. Further, glass fibers can be produced in a wide variety of fiber diameters and such fibers have a low moisture absorption as well as high strength at elevated temperatures. A high-temperature filter fabric comprised of an all glass fiber batt and mineral fiber base layer as described in the present invention, enables users of such needle-felted fabrics to reduce the cooling requirements of the auxiliary devices heretofore needed for lowering the temperature of unfiltered gases. These properties make glass fibers an ideal material for filtering, particularly at high temperatures or in most corrosive environments. However, until the advent of the present invention, it was not thought possible to needle glass fibers onto a substrate so as to produce a needle-felted filter fabric construction having the strength and durability needed in many industrial applications. We believe that prior art attempts to produce such a structure have failed due to the high breakage of the glass fibers in the batt whenever needling has been attempted. It is thought that the reason why previous attempts to produce needle-felted filter fabrics having glass fiber batts have failed is due to the well-known fact that, in needling glass fiber batts, the brittleness of the glass fibers having relatively large fiber diameters causes the disintegration of the fibers to such an extent that the resulting product is rendered unusable as a filter for many commercial applications.
We have found that by needling glass fibers having a diameter of from 2 to 8 microns onto a mineral fabric substrate and using a suitable resin-binder, it is possible to produce a needle-felted filter fabric which can filter gases continuously at temperatures as high as at least about 500.degree. Fahrenheit and intermittently at higher temperatures.
Accordingly, it is an object of the present invention to make a resin-impregnated needle-felted filter fabric which comprises a surface or batt layer comprised of glass fibers and an underlayer which is comprised of mineral fibers.
It is also an object of the present invention to make a needle-felted filter fabric which is capable of removing particles from gases at elevated temperatures.
It is another object of the present invention to make a needle-felted filter fabric having high tensile strength as well as dimensional stability at elevated temperatures.
It is a further object of the present invention to make a needle-felted filter fabric which is economical to produce as well as relatively long-lasting.
It is a still further object of the present invention to make a needle-felted filter fabric having low moisture absorption as well as resistance to most corrosive environments.
It is a still further objective of the present invention to provide a method for needling glass fibers onto a mineral fiber base layer.
Another object of the present invention is to provide a needle-felted filter fabric having a large number of pores within a relatively small area.
A further object of the present invention is to provide a needle-felted filter fabric having relatively uniform pore dimensions.
A still further object of the present invention is to provide a needle-felted filter fabric having a high air to cloth ratio.
A still further object of the present invention is to provide a needle-felted filter fabric having desirable particle release properties.
Another object of the present invention is to provide a needle-felted filter fabric having a high filtering efficiency.